<!DOCTYPE html>
<html>
	<head>
		<meta charset="utf-8">
		<title></title>
	</head>
	<body>
		
	</body>
</html>
<script>debugger;
	var a = 100;
	console.log(a,window.a); // 100 100
	let b = 10;
	console.log(b,window.b); // 10 undefined
	const c = 1;
	console.log(c,window.c); // 1 undefined
	
	console.log(d); // undefined  ===>  a已声明还没赋值，默认得到undefined值
	var d = 100;
</script>
